The ultraviolet-hydrogen peroxide (UV-HP) process has the potential to achieve complete destruction or transformation of organic contaminants, and has therefore emerged as a potentially powerful technology in water treatment. However, the complex kinetics and mechanisms associated with free-radical reactions, as well as many influencing parameters, often render the reaction engineering and process optimization difficult. The present research specifically addresses the optimization of the UV-HP process for the decomposition of endocrine disrupting chemicals (ECDs), exemplified by alachlor, an acetanilide pesticide, in groundwaters containing natural organic matter (NOM). Various influencing parameters were investigated through a systematic approach of combining experimental data acquisition with modeling techniques. A kinetic model for the UV-HP process was developed for predicting the concentrations of all principal species, including alachlor, NOM, hydrogen peroxide, carbonates, and intermediate radicals, as functions of time. The model was based on the free-radical reaction mechanisms with ordinary differential equations representing the rate expressions of the principal species. Several numerical methods including the finite backward difference formulas (Gear's methods) and the Runge-Kutta method were used to solve the model equations, and their relative computational performances were evaluated. The model predictions were satisfactorily verified by the experimental results obtained for NOM-free and NOM-containing waters.

The UV-HP oxidation batch kinetic studies and model simulations demonstrated that the alachlor decomposition efficiency decreased with increasing initial solution pH and total carbonate concentration. The reaction kinetics of alachlor decomposition were significantly hindered by the presence of NOM. Nonetheless, the process was effective in lowering alachlor concentration to below the maximum contaminant level (MCL) of 2 ƒÝg/L in an actual groundwater containing NOM. This was attributed to the high reactivity of the hydroxyl radical with the pesticide. The NOM decomposition was found to be substantially enhanced as time progressed. An explanation could be offered that progressive reduction in the species responsible for hydroxyl radical scavenging and UV shielding increased the NOM decomposition rate. These effects were satisfactorily described and predicted by the proposed model. Model simulation studies also demonstrated that the alachlor decomposition efficiency was significantly influenced by NOM and total carbonate concentrations. The proposed model accounted for the generation and consumption of free radicals associated with UV-HP oxidation, and predicted their non-steady-state concentrations, thus providing insight into the complex kinetics and reaction mechanisms. Furthermore, the model facilitated the optimization of process variables for a given water quality matrix to achieve rapid decontamination in a cost-effective manner.

Key words: Reaction engineering, ultraviolet-hydrogen peroxide oxidation, advanced oxidation, reaction modeling, water treatment, endocrine disrupting chemicals